Name: Application layer
Uploaded: 2024-02-10
Duration: 16 min 42 s
Description: *. Unit-5 : Application Layer. ‹#›. Application layer.

undefined. *. Unit-5 : Application Layer. ‹#›. Application layer. 

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[Audio] The Application Layer is focused on how user interacts with the network. It provides services to the user, including sending emails, accessing and transferring files, accessing system resources, accessing the World Wide Web, and managing the network. Additionally, it offers user interfaces to enable interaction with the network. All necessary software applications for the network are based on this layer.. 

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The application layer enables the user, whether human or software, to access the network. 
 It provides user interfaces and support for services such as electronic mail, file access and transfer, access to system resources, surfing the world wide web, and network management.
 The application layer is responsible for providing services to the user.

[Audio] The application layer of the Internet model is our focus today. It contains client/server programs, like email, which are directly accessible to users. Additionally, this layer has programs that support other applications, like the Domain Name System (DNS). This program translates URLs into IP addresses so that emails can be delivered to the right destination. An example of this process can be seen in figure 25.1. Emails are sent using Simple Mail Transfer Protocol (SMTP) and the DNS client locates the IP address of the recipient.. 

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There are several applications in the application layer of the Internet model that follow the client/server paradigm. The client/server programs can be divided into two categories: those that can be directly used by the user, such as e-mail, and those that support other application programs. The Domain Name System (DNS) is a supporting program that is used by other programs such as e-mail.

Figure 25.1 shows an example of how a DNS client/server program can support an e-mail program to find the IP address of an e-mail recipient.

[image] Figure 25. I
Example ofusing the DNS service
SMTPi Simple Mail Transfer Protocol (e-mail)
DNS: Domain Name System
U ser
aperson@wonderful.com
Application
layer
wonderful.com
200.200.2005
DNS
client
Transport layer

[Audio] Good morning everyone. Today I'll be talking about Application Layer. 25.1 talks about Name Space, specifically Flat Name Space and Hierarchical Name Space. As each address is unique, a name space must be organized in one of two ways: fiat or hierarchical. When names are created, each part of the name has a certain purpose. The first part can define the nature of the organization, the second part can define the name of the organization, and the third part can define departments within the organization.. 

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The names must be unique because the addresses are unique. A name space that maps each address to a unique name can be organized in two ways: fiat or hierarchical.

A name is assigned to an address. A name in this space is a sequence of characters without structure.

Each name is made of several parts. The first part can define the nature ofthe organization, the second part can define the name of an organiza-tion, the third part can define departments in the organization, and so on.

To have a hierarchical name space, a domain name space was designed. In this design the names are defined in an inverted-tree structure with the root at the top. The tree can have only 128 levels: level 0 (root) to level 127 (see Figure 25.2).. 

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To have a hierarchical name space, a domain name space was designed. In this design the names are defined in an inverted-tree structure with the root at the top. The tree can have only 128 levels: level 0 (root) to level 127 (see Figure 25.2).

[Audio] We are discussing the Application Layer and its components. Labels are strings with a maximum of 63 characters and the root label is empty. Domain names are a sequence of labels, as shown in the image. Each node in the tree has its own domain name. The root domain name is 'edu' and below that is 'fhda.edu', followed by 'atc.fhda.edu' and finally 'challenger.atc.fhda.edu'.. 

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Label Each node in the tree has a label, which is a string with a maximum of 63 characters. The root label is a null string (empty string). 

 Domain Name Each node in the tree has a domain name. A full domain name is a sequence of labels Figure 25.3 shows some domain names.

[image] edu.
Root
Domain name
Label
edu
abel
fhda
abel
atc
challenger
fhda.edu. Domain name
atc.fhda.edu. Domain name
challenger.atc.fhda.edu. Domain name

[Audio] The application layer is the top layer of the TCP/IP reference model. At this layer, information is formatted into what is known as user datagrams or, more commonly, packets. The purpose of this layer is to provide an interface between the user and the network, allowing applications to use the network to send and receive data. In this unit, we will look at the protocols that make up the application layer, as well as how they are used to facilitate communication between different nodes on the network.. 

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A domain is a subtree of the domain name space. The name of the domain is the domain name of the node at the top of the subtree. Figure 25.5 shows some domains. Note that a domain may itself be divided into domains (or subdomains as they are sometimes called).

[Audio] The Domain Name Space is a hierarchical tree-like structure used to organize domains on the internet. It is divided into a number of zones and each zone is managed by a Root Server. To effectively manage such a vast amount of information, the Domain Name Space needs to be distributed across multiple computers. This not only reduces the workload of the Root Server but also ensures that the information is more reliable, as a single failure does not make the entire tree inaccessible.. 

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The information contained in the domain name space must be stored. 
 However, it is very inefficient and also unreliable to have just one computer store such a huge amount of information. 
 It is inefficient because responding to requests from all over the world places a heavy load on the system. 
 It is not unreliable because any failure makes the data inaccessible.

[Audio] Inputs like mouse, keystrokes, and clicks are part of User Interface at the Application Layer. Outputs encompass text, video, images, and sound. The Domain Name System (DNS) is used to assign a logical address to the user. DNS helps to manage the size and complexity of the global network by disseminating the information among different computers, therefore making it easier for users to access the required services by using logical addresses.. 

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The solution to these problems is to distribute the information among many computers called DNS servers.

[Audio] At the top of the OSI model is the Application layer, which acts as a bridge between the user and the network. It offers services such as e-mail, file transfer, and web-based applications, and allows users to access network resources and receive services from the network. Protocols used to provide these services include HTTP, FTP, and SMTP.. 

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[Audio] In this slide, we will be looking at application layer and focusing on the concept of Primary and Secondary server. A primary server is a computer that sends data or services as requested by a client. A Secondary server, on the other hand, receives requests from a primary server. It loads all the information from the primary server. It should be noted that the primary server obtains all the information from the disk file. We will be discussing this further in the upcoming slides.. 

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A primary server loads all information from the disk file; the secondary server loads all information from the primary server.

[Audio] The Domain Name System (DNS) is a protocol used to map domain names to IP addresses. The domain name space is divided into three sections: generic domains, country domains, and inverse domains. Inverse domains are set up in a hierarchical tree structure moving from the root up to the country domains and then to the generic domains. This structure allows for faster and more reliable service by allowing domains to be located more accurately.. 

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DNS is a protocol that can be used in different platforms. In the Internet, the domain name space (tree) is divided into three different sections: generic domains, country domains, and the inverse domain

[image] Inverse domain
Root
Generic domains
Country domains

[Audio] The application layer divides the domain name space into the top-level domains. At the top of the tree is the root domain. Moving down the tree there are the top-level domains divided into two categories: Generic domains and Country code domains. The generic domains define registered hosts according to their generic behavior. Each node in the tree defines a domain, which is an index to the domain name space database.. 

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[image] gov
Root level
int
com
fhda •••
edu
atc
chal •
mil
net
chal.atc.fhda.edu.
Index to address
org
Generic
domains

The generic domains define registered hosts according to their generic behavior. 

 Each node in the tree defines a domain, which is an index to the domain name space database

[Audio] At the Application Layer of the network model, applications interact directly with the network. This is where the Label Description Format (LDF) comes into play. LDF is an IP addressing system which assigns labels to organizations based on their type. The eight different categories of labels are 'com' for commercial, 'edu' for educational, 'gov' for government, 'int' for international, 'mil' for military, 'net' for network support, and 'org' for nonprofit organizations. This system allows for unique identification of organizations across the internet.. 

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Label Description com Commercial organizations edu Educational institutions gov Government institutions int International organizations mil Military groups net Network support centers org Nonprofit organizations

[Audio] The Application Layer of the OSI model is the layer that interacts directly with software applications that use the network. This layer contains information about the type of network services and protocols used. A table can be seen below that labels and describes common application layer protocols which includes Airlines and Aerospace companies, Businesses and Firms, Cooperatives, Information Service Providers, Museums and Nonprofits, Personal Names, and Professional Individual Organizations.. 

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Label Description aero Airlines and aerospace companies biz Businesses or firms (similar to com) coop Cooperative business organizations info Information service providers museum Museums and other nonprofit organizations name Personal names (individuals) pro Professional individual organizations

[Audio] The application layer of an address can be broken down into components to locate individual computers on the network. It starts with a root level, such as us.ca.cup.anza.fr, which can be divided into an index and country domain sections. The index is used to point to subdomains, like us.ca.cup that can be further subdivided into zw and anza. The country domain section uses two-character country abbreviations, like us for the United States. Second-level labels refer to organizations or more specific, national designations, like using abbreviations to subdivide the us designation.. 

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[image] Root level
fr
anza.cup.ca.us.
Index to address
zw
cup
anza
Country domai

The country domains section uses two-character country abbreviations (e.g., us for United States). 
 Second labels can be organizational, or they can be more specific, national designations.The United States, for example, uses state abbreviations as a subdivision of us

[Audio] The Application layer of the TCP/IP protocol stack is responsible for the exchange of data between applications. Inverse domains can be used to map an IP address to its corresponding domain name. This process involves reversing the IP address, for example, the inverse of 132.45.34.121 is 121.34.45.132.in-addr.arpa, which is then used to identify the resources available and who provides them.. 

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[image] arpa
in-addr
132
34
45
121
Inverse domain
Root level
1 21.45.34.132 .in-addr.arpa.
Index to name

The inverse domain is used to map an address to a name. This may happen, 
 for example, when a server has received a request from a client to do a task.

[Audio] The Application Layer is part of the OSI Model. It allows end-user software programs like web browsers, email clients, etc. access to network services. This unit covers the Domain Name System (DNS) and the messages it uses for communication. DNS is responsible for translating domain names to IP addresses at the network layer. There are two types of DNS messages: query and response. The format of both types of messages is the same - they consist of headers and question records. The response message includes additionally answer records, authoritative records, and additional records.. 

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Figure 25.13   DNS MESSAGES  (Query and response messages)

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DNS has two types of messages: query and response. Both types have the same format. The query message consists of a header and question records; the response message
 consists of a header, question records, answer records, authoritative records, and addi- tional records

[Audio] The Application Layer of the TCP/IP Model is the topmost layer of the model and the network applications interact with this layer. In the header, the Identification is a 16-bit identifier assigned to the query which is echoed in the response. The flags and numbers of questions, answers, and additional records fields are also present in the header.. 

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[image] 2 bytes
Identification
Number of question records
Number of authoritative records
(All 0s in query message)
2 bytes
Flags
Number of answer records
(All 0s in query message)
Number of additional records
(All 0s in query message)

Both query and response messages have the same header format with some fields set to zero for the query messages. The header is 12 bytes, and its format is shown in the Figure.

[Audio] DNS is the protocol used at the application layer to resolve host names to IP addresses. It can use either UDP or TCP, with a well-known port number of 53. Generally, DNS requests and answers are small, so the overhead from the higher layers is minimal.. 

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DNS can use the services of 
UDP or TCP, 
using the well-known port 53.

[Audio] The Application Layer in the OSI model allows us to access data on the World Wide Web. The most widely used protocol in this layer is Hypertext Transfer Protocol (HTTP), which incorporates functionalities from both File Transfer Protocol (FTP) and Simple Mail Transfer Protocol (SMTP), making it more efficient. Whereas FTP requires multiple TCP connections, HTTP only requires one, thus making the communication faster. The data is transmitted in the form of SMTP messages, and the messages include headers similar to MIME, which are read and comprehended by both the browser and the server, enabling us to quickly access data over the internet.. 

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The Hypertext Transfer Protocol (HTTP) is a protocol used mainly to access data on the World Wide Web. HTTP functions as a combination of FTP and SMTP. It is similar to FfP because it transfers files and uses the services of TCP. 
 However, it is much simpler than FfP because it uses only one TCP connection. There is no separate control connection; only data are transferred between the client and the server.
 HTTP is like SMTP because the data transferred between the client and the server look like SMTP messages. In addition, the format of the messages is controlled by MIME-like headers. Unlike SMTP, the HTTP messages are not destined to be read by humans; they are read and interpreted by the HTTP server and HTTP client (browser).
 SMTP messages are stored and forwarded, but HTTP messages are delivered immedi- ately. The commands from the client to the server are embedded in a request message.
 The contents of the requested file or other information are embedded in a response message.

HTTP uses the services of TCP on well-known port 80.

[Audio] The figure illustrates how client and server communicate through the stateless protocol HTTP. The client initiates the process by sending a request message to the server, which is then answered by a response message from the server. This completes the exchange with both roles being fulfilled.. 

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Figure 27.12 illustrates the HTTP transaction between the client and server. Although HTTP uses the services ofTCP, HTTP itselfis a stateless protocol. The client initializes the transaction by sending a request message. The server replies by sending a response.

[image] Server
Client
This is
a request.
This is
a response.

[Audio] At the Application Layer, requests from the user are formatted into messages which are sent to the Network Layer. The Network Layer forwards the messages to a remote application, and the response is returned from the Application Layer to the user. Request messages consist of a request line, header, and sometimes a body; response messages consist of a status line, header, and sometimes a body.. 

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Messages -> The formats of the request and response messages are similar; both are shown in Figure 27.13. A request message consists of a request line, a header, and sometimes a body.
 A response message consists of a status line, a header, and sometimes a body.

[image] Status line
Headers
A blank line
Body
(present only in
some messages)
Response message

[Audio] Good morning everyone. In this slide, we will take a look at the application layer of a computer network and its components. In this layer, Request Type, or in other words the method, is used to initiate an action on the server, such as GET, POST, or PUT. After the Request Type is the Uniform Resource Locator URL, which is the address of the resource on the server. Following this is the HTTP version, which is used to tell the server what version of the HTTP protocol is being used. Finally, in the request and status lines, the common field is the HTTP version which is then followed by the status code and the status phrase.. 

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[image] Space
Request type
(method)
URL
Space
HTTP version

Request and Status Lines The first line in a request message is called a request line; the first line in the response message is called the status line. There is one common field, as shown in Figure 27.14.

[image] Space
HTTP version
Space
Status code
Status phrase

[Audio] In this slide we discuss the application layer, the highest layer of the OSI reference model. It provides services to applications like web browsers and web servers. In this layer, a header exchange occurs to send and receive extra information about the document, such as the header name and its value. That is all for this slide.. 

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Header exchanges additional information between the client and the server.
 For example, the client can request that the document be sent in a special format, or the server can send extra information about the document. The header can consist of one or more header lines. Each header line has a header name, a colon, a space, and a header value

[Audio] In this slide, the application layer is responsible for the communication between the applications that take the requests and the format of the information that is exchanged. To further illustrate this, let's look at the structure of a request and a response message. Both of them consist of headers, a blank line, and a body. The request message includes a request line, general headers, request headers, entity headers, and a body. On the other hand, the response message contains a status line, general headers, response headers, entity headers, and a body.. 

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[image] Request line
General headers
Request headers
Entity headers
A blank line
Body
Request message
Status line
General headers
Response headers
Entity headers
A blank line
Body
Response message

[Audio] In this section, we will go over the application layer and its use in retrieving a document. To demonstrate this, we will use the GET method to request for an image with the path /usr/bin/image1. The request line consists of the method, the URL, and the HTTP version. The header lines indicate that the client is able to accept images in GIF and JPEG format, and the response message contains the status line and header lines which define the date, server, MIME version, length of the document. The body of the document is at the end. This will help us understand how the application layer works and allows clients to access documents.. 

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This example retrieves a document. We use the GET method to retrieve an image with the path /usr/bin/image1. The request line shows the method (GET), the URL, and the HTTP version (1.1). The header has two lines that show that the client can accept images in GIF and JPEG format. The request does not have a body. The response message contains the status line and four lines of header. The header lines define the date, server, MIME version, and length of the document. The body of the document follows the header (see Fig. 27.9, next slide).

[Audio] The Application layer provides the interface between user applications and the transport layer. A client sends a request using the GET method to the server, which checks the requested resource. The server then sends the response using the HTTP/1.1 protocol including the content length and MIME-version, followed by the body of the requested document.. 

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[image] Server
Client
Request (GET method)
GET /usr/bin/imagel HTTP/ 1.1
Accept: image/gif
Accept: image/jpeg
Response
HTTP/I.1 200 0K
Date: Mon, 07-Jan-02 GMT
Server: Challenger
MIME-version: 1.0
Content-length: 2048
(Body of the document)

[Audio] An example of the HTTP protocol, part of the application layer, is looked at in this slide. The HEAD method is used to send a request to a web server to obtain information pertaining to an HTML document. This request consists of a status line and five lines of header. The date, server, MIME version, type of document, and length of the document are the return values. Pay attention that the response does not incorporate a body.. 

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This example retrieves information about a document. We use the HEAD method to retrieve information about an HTML document (see the next section). The request line shows the method (HEAD), URL, and HTTP version (1.1). The header is one line showing that the client can accept the document in any format (wild card). The request does not have a body. The response message contains the status line and five lines of header. The header lines define the date, server, MIME version, type of document, and length of the document (see Fig. 27.10, next slide). Note that the response message does not contain a body.

Figure 27.10 Example 2. [image] Server Client Request (HEAD method) HEAD /usr/user/filel.html HITP/I.I Accept: Response HTTP/I.1 200 0K Date: Tue, 08-Jan-02 GMT Server: Challenger MIME-version: 1.0 Content-type: text/html Content-length: 1020. 

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[image] Server
Client
Request (HEAD method)
HEAD /usr/user/filel.html HITP/I.I
Accept:
Response
HTTP/I.1 200 0K
Date: Tue, 08-Jan-02 GMT
Server: Challenger
MIME-version: 1.0
Content-type: text/html
Content-length: 1020

[Audio] We will discuss nonpersistent and persistent connections. Nonpersistent connection involves establishing a TCP connection for each request-response, while persistent connection lets the server keep the connection open after sending out response, thus avoiding the need for re-establishing the connection. Generally, the sender sends the length of the data with each response, unless the document is created dynamically or actively. In these cases, the server informs the client that the length is not known and closes the connection after sending the data.. 

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Persistent Versus Nonpersistent Connection

Nonpersistent Connection In a nonpersistent connection, one TCP connection is made for each request/response.
 The following lists the steps in this strategy: 1. The client opens a TCP connection and sends a request. 2. The server sends the response and closes the connection. 3. The client reads the data until it encounters an end-of-file marker; it then closes the connection.

Persistent Connection HTTP version 1.1 specifies a persistent connection by default. In a persistent connection, the server leaves the connection open for more requests after sending a response.  The server can close the connection at the request of a client or if a time-out has been reached.
 The sender usually sends the length of the data with each response. However, there are some occasions when the sender does not know the length of the data. This is the case when a document is created dynamically or actively. 
 In these cases, the server informs the client that the length is not known and closes the connection after sending the data so the client knows that the end of the data has been reached.

HTTP version 1.1 specifies a persistent connection by default.

END. *. ‹#›. Unit-5 : Application Layer. 

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Application layer

Scene 0

The Application Layer is focused on how user interacts with the network. It provides services to the user
 including sending emails
 accessing and transferring files
 accessing system resources
 accessing the World Wide Web
 and managing the network. Additionally
 it offers user interfaces to enable interaction with the network. All necessary software applications for the network are based on this layer.

avatar

The application layer of the Internet model is our focus today. It contains client/server programs
 like email
 which are directly accessible to users. Additionally
 this layer has programs that support other applications
 like the Domain Name System (D-N-S--). This program translates U-R-L--s into IP addresses so that emails can be delivered to the right destination. An example of this process can be seen in figure 25.1. Emails are sent using Simple Mail Transfer Protocol (S-M-T-P-) and the D-N-S-- client locates the IP address of the recipient.

Good morning everyone. Today I'll be talking about Application Layer.  25.1 talks about Name Space
 specifically Flat Name Space and Hierarchical Name Space. As each address is unique
 a name space must be organized in one of two ways: fiat or hierarchical. When names are created
 each part of the name has a certain purpose. The first part can define the nature of the organization
 the second part can define the name of the organization
 and the third part can define departments within the organization.

We are discussing the Application Layer and its components. Labels are strings with a maximum of 63 characters and the root label is empty. Domain names are a sequence of labels
 as shown in the image. Each node in the tree has its own domain name. The root domain name is 'edu' and below that is 'fhda.edu'
 followed by 'atc.fhda.edu' and finally 'challenger.atc.fhda.edu'.

The application layer is the top layer of the T-C-P/IP reference model. At this layer
 information is formatted into what is known as user datagrams or
 more commonly
 packets. The purpose of this layer is to provide an interface between the user and the network
 allowing applications to use the network to send and receive data. In this unit
 we will look at the protocols that make up the application layer
 as well as how they are used to facilitate communication between different nodes on the network.

The Domain Name Space is a hierarchical tree-like structure used to organize domains on the internet. It is divided into a number of zones and each zone is managed by a Root Server. To effectively manage such a vast amount of information
 the Domain Name Space needs to be distributed across multiple computers. This not only reduces the workload of the Root Server but also ensures that the information is more reliable
 as a single failure does not make the entire tree inaccessible.

Inputs like mouse
 keystrokes
 and clicks are part of User Interface at the Application Layer. Outputs encompass text
 video
 images
 and sound. The Domain Name System (D-N-S--) is used to assign a logical address to the user. D-N-S-- helps to manage the size and complexity of the global network by disseminating the information among different computers
 therefore making it easier for users to access the required services by using logical addresses.

At the top of the O-S-I-- model is the Application layer
 which acts as a bridge between the user and the network. It offers services such as e-mail
 file transfer
 and web-based applications
 and allows users to access network resources and receive services from the network. Protocols used to provide these services include H-T-TP
 F-T-P--
 and S-M-TP.

In this slide
 we will be looking at application layer and focusing on the concept of Primary and Secondary server. A primary server is a computer that sends data or services as requested by a client. A Secondary server
 on the other hand
 receives requests from a primary server. It loads all the information from the primary server. It should be noted that the primary server obtains all the information from the disk file. We will be discussing this further in the upcoming slides.

The Domain Name System (D-N-S--) is a protocol used to map domain names to IP addresses. The domain name space is divided into three sections: generic domains
 country domains
 and inverse domains. Inverse domains are set up in a hierarchical tree structure moving from the root up to the country domains and then to the generic domains. This structure allows for faster and more reliable service by allowing domains to be located more accurately.

The application layer divides the domain name space into the top-level domains. At the top of the tree is the root domain. Moving down the tree there are the top-level domains divided into two categories: Generic domains and Country code domains. The generic domains define registered hosts according to their generic behavior. Each node in the tree defines a domain
 which is an index to the domain name space database.

At the Application Layer of the network model
 applications interact directly with the network. This is where the Label Description Format (L-D-F--) comes into play. L-D-F-- is an IP addressing system which assigns labels to organizations based on their type. The eight different categories of labels are 'com' for commercial
 'edu' for educational
 'gov' for government
 'int' for international
 'mil' for military
 'net' for network support
 and 'org' for nonprofit organizations. This system allows for unique identification of organizations across the internet.

The Application Layer of the O-S-I-- model is the layer that interacts directly with software applications that use the network. This layer contains information about the type of network services and protocols used. A table can be seen below that labels and describes common application layer protocols which includes Airlines and Aerospace companies
 Businesses and Firms
 Cooperatives
 Information Service Providers
 Museums and Nonprofits
 Personal Names
 and Professional Individual Organizations.

The application layer of an address can be broken down into components to locate individual computers on the network. It starts with a root level
 such as us.ca.cup.anza.fr
 which can be divided into an index and country domain sections. The index is used to point to subdomains
 like us.ca.cup that can be further subdivided into zw and anza. The country domain section uses two-character country abbreviations
 like us for the United States. Second-level labels refer to organizations or more specific
 national designations
 like using abbreviations to subdivide the us designation.

The Application layer of the T-C-P/IP protocol stack is responsible for the exchange of data between applications. Inverse domains can be used to map an IP address to its corresponding domain name. This process involves reversing the IP address
 for example
 the inverse of 132.45.34.121 is 121.34.45.132.in-addr.arpa
 which is then used to identify the resources available and who provides them.

The Application Layer is part of the O-S-I Model. It allows end-user software programs like web browsers
 email clients
 et cetera access to network services. This unit covers the Domain Name System (D-N-S--) and the messages it uses for communication. D-N-S-- is responsible for translating domain names to IP addresses at the network layer. There are two types of D-N-S-- messages: query and response. The format of both types of messages is the same - they consist of headers and question records. The response message includes additionally answer records
 authoritative records
 and additional records.

The Application Layer of the T-C-P/IP Model is the topmost layer of the model and the network applications interact with this layer. In the header
 the Identification is a 16-bit identifier assigned to the query which is echoed in the response. The flags and numbers of questions
 answers
 and additional records fields are also present in the header.

D-N-S-- is the protocol used at the application layer to resolve host names to IP addresses. It can use either U-D-P-- or T-C-P--
 with a well-known port number of 53. Generally
 D-N-S-- requests and answers are small
 so the overhead from the higher layers is minimal.

The Application Layer in the O-S-I-- model allows us to access data on the World Wide Web. The most widely used protocol in this layer is Hypertext Transfer Protocol (H-T-T-P-)
 which incorporates functionalities from both File Transfer Protocol (F-T-P--) and Simple Mail Transfer Protocol (S-M-T-P-)
 making it more efficient. Whereas F-T-P-- requires multiple T-C-P-- connections
 H-T-T-P- only requires one
 thus making the communication faster. The data is transmitted in the form of S-M-T-P- messages
 and the messages include headers similar to M-I-M-E-
 which are read and comprehended by both the browser and the server
 enabling us to quickly access data over the internet.

The figure illustrates how client and server communicate through the stateless protocol H-T-TP. The client initiates the process by sending a request message to the server
 which is then answered by a response message from the server. This completes the exchange with both roles being fulfilled.

At the Application Layer
 requests from the user are formatted into messages which are sent to the Network Layer. The Network Layer forwards the messages to a remote application
 and the response is returned from the Application Layer to the user. Request messages consist of a request line
 header
 and sometimes a body; response messages consist of a status line
 header
 and sometimes a body.

Good morning everyone. In this slide
 we will take a look at the application layer of a computer network and its components. In this layer
 Request Type
 or in other words the method
 is used to initiate an action on the server
 such as G-E-T
 P-O-S-T-
 or P-U-T. After the Request Type is the Uniform Resource Locator U-R-L--
 which is the address of the resource on the server. Following this is the H-T-T-P- version
 which is used to tell the server what version of the H-T-T-P- protocol is being used. Finally
 in the request and status lines
 the common field is the H-T-T-P- version which is then followed by the status code and the status phrase.

In this slide we discuss the application layer
 the highest layer of the O-S-I-- reference model. It provides services to applications like web browsers and web servers. In this layer
 a header exchange occurs to send and receive extra information about the document
 such as the header name and its value. That is all for this slide.

In this slide
 the application layer is responsible for the communication between the applications that take the requests and the format of the information that is exchanged. To further illustrate this
 let's look at the structure of a request and a response message. Both of them consist of headers
 a blank line
 and a body. The request message includes a request line
 general headers
 request headers
 entity headers
 and a body. On the other hand
 the response message contains a status line
 general headers
 response headers
 entity headers
 and a body.

In this section
 we will go over the application layer and its use in retrieving a document. To demonstrate this
 we will use the G-E-T-- method to request for an image with the path /usr/bin/image1. The request line consists of the method
 the U-R-L--
 and the H-T-T-P- version. The header lines indicate that the client is able to accept images in G-I-F-- and J-P-E-G- format
 and the response message contains the status line and header lines which define the date
 server
 M-I-M-E- version
 length of the document. The body of the document is at the end. This will help us understand how the application layer works and allows clients to access documents.

The Application layer provides the interface between user applications and the transport layer. A client sends a request using the G-E-T-- method to the server
 which checks the requested resource. The server then sends the response using the H-T-TP/1.1 protocol including the content length and M-I-ME-version
 followed by the body of the requested document.

An example of the H-T-T-P- protocol
 part of the application layer
 is looked at in this slide. The H-E-A-D- method is used to send a request to a web server to obtain information pertaining to an H-T-M-L- document. This request consists of a status line and five lines of header. The date
 server
 M-I-M-E- version
 type of document
 and length of the document are the return values. Pay attention that the response does not incorporate a body.

We will discuss nonpersistent and persistent connections. Nonpersistent connection involves establishing a T-C-P-- connection for each request-response
 while persistent connection lets the server keep the connection open after sending out response
 thus avoiding the need for re-establishing the connection. Generally
 the sender sends the length of the data with each response
 unless the document is created dynamically or actively. In these cases
 the server informs the client that the length is not known and closes the connection after sending the data.